Diamond materials have a large variety of applications ranging from gems to industrial application such as semiconductor devices, abrasive tools, and optical devices. Large diamonds can be made by conventional chemical vapor deposition (CVD) methods but these approaches are very slow and costly for more than a few microns in thickness. On the other hand, larger diamond particles have been made by using ultrahigh pressure apparatuses (e.g. belt apparatus, cubic press) with pressures up to several gigapascals under largely solid phase growth of diamond. Due to volume limitations inherent in these devices, the throughput has been low (e.g. one crystal per machine) and the cost has been high. In the case of ultrahigh pressure devices, diamond can grow relatively fast (>1 mm per hour) which is about one order of magnitude higher than CVD diamond.
Conventional CVD diamond film is deposited under partial vacuum at about 900° C. Normally, graphite with sp2 hybridization should be the stable phase, but because of the presence of hydrogen atoms (not molecules of H2), the decomposed carbon atoms from carbonaceous gas (e.g. methane) is bombarded with hydrogen atoms to maintain its diamondoid structure (sp3 hybridization). The growth rate and the quality of CVD diamond are highly dependent on the concentration of hydrogen atoms.
Unfortunately, hydrogen atoms are made by dissociation of hydrogen molecules. This requires a high temperature (>2000° C.), while the mean free path of dissociated hydrogen atoms is short, such that most hydrogen atoms recombine to form molecules to release heat. Consequently, the concentration of hydrogen atoms near the substrate is low. And hence, the growth of diamond film is slow (a few microns per hour). This problem cannot be solved by merely moving the substrate closer to the heat source as graphite will be formed at high temperatures. Thus, conventional CVD diamond processes have a dilemma that high temperature boosts the production of hydrogen atoms, but diamond must be deposited at lower temperature where hydrogen atoms have recombined.